In homogeneous catalysis, many catalyst systems comprising a precious metal find application, usually in the form of a soluble complex of the metal. Generally, homogeneous catalysis provides advantages over heterogeneous catalysis by higher reaction rates per atomic amount of metal and higher selectivities. On the other hand, heterogeneous catalysis allows for a much more easy separation of catalyst and reaction medium, whereby heterogeneous catalysts can readily be reused or used continuously. Homogeneous catalysts tend to show a shorter lifetime.
Unless the reaction product, for example being gaseous, can continuously be stripped from a stationary reaction medium, the homogeneous catalyst will pass through and leave a reactor with the reaction effluent. When using a stripping reactor, usually side reactions will proceed that lead to the formation of non-volatile substances, for which reason application of a reactor bleed is necessary to prevent accumulation of these non-volatiles in the reaction mixture. By the bleed effluent some of the precious metal will leave the reactor.
Some catalysts constitute stable precious metal complexes and may be readily extracted from the reaction effluent for recycle, if a solubilizing fluid immiscible with the reaction medium is available. Some other catalysts can readily be reconstituted from their precursors extracted from the reaction effluent. However, many catalysts are being degraded during the reaction cycle or subsequent work-up, and the reaction effluent may comprise various remnants of the precious metal catalyst with various valency states for the metal, so that recycling is not possible. In other cases the presence of excessive contaminants will preclude recycling. In view of costs of the precious metal and considering the environmentally stipulated restrictions on disposal of heavy metal-containing waste, effective recovery of the residual precious metal from the reaction effluent is of paramount importance for an economically viable process using a homogeneous precious metal catalyst.
Several methods for recovering the precious metal have already been proposed. According to Great Britain A-2127001 precious metals are rapidly and efficiently recovered from cyanide-containing leach solutions by loading onto an activated carbon fibre body, while CS-B-251467 [Chem.Abstr., 109(24): 213964y] teaches the recovery of palladium catalyst from acidified waste water by sorption on activated carbon pretreated with an alkali metal salt of EDTA. However, these methods concern aqueous waste liquids, and are not directly applicable to non-aqueous systems.
JP-A-196537/1988 and JP-A-197543/1988 disclose an adsorbent which is constituted by oxidized activated carbon loaded with organophosphorus compounds for reversible adsorption and desorption of Group VIII noble metal complexes, for example in hydroformylation reactions. This method requires specific properties of the catalytic metal complex being sufficiently stable for direct reuse, and accordingly has limited applicability. JP-A-231630/1985 discloses the use of activated carbon for the recovery of palladium compounds from the reaction effluent of the oxidative carbonylation of styrene conducted in the presence of molecular oxygen and oxidants such as ferric and cupric salts. Under such conditions the palladium is essentially present in its +2 valency state and any salts present are simple inorganic salts. It has appeared, that this method is not completely effective, if non-aqueous effluents of more complicated composition are treated.
It is also known to recover precious metal ions using an ion-exchange resin, but this method will not effect essentially complete recovery, if not all precious metal is present in the ionic state or too much of contaminants is present in the effluents to be treated.
U.S. Pat. No. 4,791,190 discloses a process for the removal of palladium catalyst residues from polymer ketones, wherein a suspension of the polymer is contacted with carbon monoxide for solubilizing and extracting the catalyst residue from the solid polymer particles.
In summary, though some processes are known for the recovery of precious metal catalysts from specific catalyst residues, there remains a need for a versatile method for recovery of precious metal catalyst waste from non-aqueous reaction effluents, in particular if the reaction effluent contains salt and/or polyether contaminants, or the precious metal is present as a plurality of complexes or in a plurality of valency states, or any combination of such complications occurs.